Detect extcon device type from cable name.

Kernel modified the extcon device name to extcon[X] for sysfs instead of
separate device name. Follow that kernel change, the new
ExtconUEventObserver will retrieve extcon device type from
/sys/class/extcon/extcon[X]/cable.[Y]/name.

The target device needs to label the SoC's extcons to sysfs_extcon in
the device's vendor sepolicy to allow the system_server access.

Bug: 152245127
Bug: 193492798
Bug: 193114615
Test: presubmit and manual tests
Change-Id: I3468acc1ef7a273da7b81a223c884079bf465a9e